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(1) Field of the Invention
The present invention generally relates to conductive adhesives suitable for use in making electrical connections in electronic assemblies. More particularly, this invention relates to a conductive adhesive material whose composition enables electrically-conductive particles to be metallurgically bonded to each other and to metal surfaces contacted by the adhesive material during bonding, thereby promoting the electrical continuity and structural integrity of the resulting electrical connection.
(2) Description of the Related Art
Conductive adhesives (CA) have been used in electronic assemblies to make electrical connections, such as attaching components to circuit boards and within flexible and rigid circuit boards. Conductive adhesives can be categorized as isotropic (ICA) or anisotropic (ACA), the latter of which is also known as a z-axis conductive adhesive. ICA""s generally comprise a polymer matrix of a thermosetting or thermoplastic material in which is dispersed small conductive particles that may be metal coated or formed entirely of metal. In ACA""s, the conducting particles are typically solid metal or polymer spheres ultimately coated with a noble metal, usually gold. In a typical ICA application, a conductive adhesive is dispensed so as to be between a pair of terminals, such as a lead of a component and a trace on a substrate, and then heated to cure the polymer matrix, forming an interconnect that bonds and electrically connects the lead with the trace. In a typical ACA application, a film or paste adhesive with randomly dispersed conducting particles is tacked, printed or dispensed onto a substrate throughout the entire contact area.
Conductive adhesives suffer from several shortcomings, one of which is that the adhesive strength of existing conductive adhesives is generally inadequate to withstand mechanical shocks that can occur during the assembly process or in service. Another and more limiting disadvantage of conductive adhesives is that the conductive path through the interconnect is defined by the conductive particles that physically contact each other, but are bonded to each other and to the terminals by the polymer matrix of the adhesive. FIGS. 1 and 2 represent two types of interconnections that illustrate this shortcoming. In FIG. 1, an isotropic conductive adhesive 112 is shown as adhering the lead 114 of a surface-mount component (not shown) to a metal trace 116 on a laminate substrate 118, forming an electrical interconnect 110. The conductive adhesive 112 conventionally contains metal particles 120 dispersed in an adhesive matrix 122. The particles 120 are maintained in physical contact with each other, the lead 114 and the trace 116 by only the adhesive matrix 122. Consequently, the robustness of the electrical interconnect 110 is not determined by the conductivity of the individual particles 120, but instead by the interfacial conductivity between the particles 120 and between the particles 120, lead 114 and trace 116. Because the particles 120 inevitably oxidize due to oxygen and/or moisture intrusion into the adhesive matrix 122, an oxide layer is typically present at the interfacial surfaces of the particles 120. For this reason, the particles 120 are typically formed from a material whose oxide is conductive, such as silver. However, silver is expensive and silver migration is detrimental to the integrity of the electrical connection. Regardless of what material the particles 120 are formed of, the resulting conductive path through the interconnect 110 is not mechanically robust.
FIG. 2 illustrates the use of an anisotropic conductive adhesive 212, such as in the manufacture of a flat panel display. The adhesive 212 is shown as forming an interconnect 210 between an oxide-free metal post 214 on an I/O pad of a silicon chip 215 to an input/output trace 216 on a glass substrate 218. The conductive adhesive 212 contains conductive particles 220 (typically nickel or gold-coated polymer spheres or solid metals) dispersed in an adhesive matrix 222. As represented in FIG. 2, the standoff is on the order of the diameter of the particles 220 (made possible because the surfaces of the chip 215 and glass substrate 218 are extremely flat). As a result, interparticle resistance is not an issue with the interconnect 210. However, as with the isotropic interconnect 110 of FIG. 1, the particles 220 of the anisotropic interconnect 210 are maintained in physical contact with the metal post 214 and trace 216 by only the adhesive matrix 222. Consequently, the robustness of the electrical interconnect 210 is again primarily determined by the interfacial conductivity between each particle 220 and its corresponding die and metal substrate contacts. Any swelling of the adhesive matrix 222 or oxygen or moisture intrusion can reduce or interrupt the physical contact between the individual particles 220 and the die and substrate metals, increasing the electrical resistance of the interconnect 210 above that allowed for the application.
In view of the above, one can appreciate that in a relatively hostile environment, the interconnects 110 and 210 may electrically open or increase in resistance as a result of degradation or failure of the mechanical bond provided by their polymer adhesive matrices 122 and 222, such that device functionality can be compromised. A more robust interconnect system could be obtained by substituting the interparticle mechanical bonds with metallurgical bonds. However, previous attempts to use fusible alloys as the conductive particles of a conductive adhesive have been unsuccessful because of the aforementioned oxidation of the particles 122 and 222 which, in addition to significantly increasing the electrical resistance of the interconnects 110 and 210, prevents the particles 122 and 222 from wetting each other and the terminals when heated above the melting or solidus temperature of the particles 122 and 222. Without wetting, true metallurgical bonds cannot be obtained.
In view of the above, it can be appreciated that existing conductive adhesives are not suited for harsh environments as a result of their limited adhesive strength being inadequate to withstand mechanical shock and provide a robust electrical interconnect. Accordingly, it would be desirable of an electrical interconnect material were available to overcome the shortcomings of the prior art.
The present invention provides a conductive adhesive material and a process by which the conductive adhesive material is used to produce an electrical interconnection characterized by metallurgical bonds between electrically-conductive particles, instead of the mechanical bonds of prior art conductive adhesives. The metallurgical bonds formed between conductive particles yields a more robust electrical interconnection that can more readily withstand mechanical shocks typical of assembly processes and operating environments.
According to the invention. the conductive adhesive material comprises the conductive particles and a polymer material in which the particles are dispersed. At least the outer surfaces of the conductive particles are formed of a fusible material, and the polymer material contains a fluxing component capable of reducing metal oxides on the surfaces of the particles. As a result, heating of the oxide-free particles to a temperature at which the fusible material is at least partially molten, e.g., near the melting temperature or above the solidus temperature of the fusible material, causes the particles to metallurgically bond to each other and to metal surfaces contacted by the adhesive material. The metallurgical bonds between particles provide superior electrical continuity and structural integrity as compared to the mechanical bonds provided by prior art conductive adhesives.
In view of the above, the present invention provides for forming an electrical interconnect by dispensing the conductive adhesive material so that the adhesive material is between two electrically-conductive members. The adhesive material is then sufficiently heated to cause metal oxides on the particles to be reduced and to at least partially melt the fusible metal that forms at least the outer surfaces of the particles. On cooling the conductive adhesive material, the particles are metallurgically bonded to each other and the electrically-conductive members. The assembly process can be conveniently separated into three events, namely, die placement, metallurgical joint formation, and polymer material cure/bond. The metallurgical joint formation may occur prior to polymer material cure/bond, or these two events can occur simultaneously, depending on the particular materials used for the fusible and polymer materials.
According to the invention, the metallurgical bonds formed between conductive particles enable the conductive adhesive material to withstand mechanical shocks typical of assembly processes and operating environments as a result of the improved strength provided by the metallurgical bonds as compared to the mechanical bonds of prior art conductive adhesives. The metallurgical bonds between particles also provide a more robust electrical interconnect that is capable of maintaining electrical continuity at higher current loads than that possible with a prior art conductive adhesive containing a similar amount of conductive particles. The metallurgical bonds also assuage degradation of electrical continuity caused by corrosion that occurs at the particle surfaces, including the interfacial particle surfaces of prior art conductive adhesives, as a result of the permeability of adhesive materials to oxygen and moisture and the ease of oxidation at the interface between abrading particles. Finally, because the continuous conductive path formed by the metallurgically-bonded particles is encapsulated by the polymer material, electrical interconnects formed with the conductive adhesive material of this invention are capable of exhibiting improved fatigue life as compared to conventional interconnections formed by solder alloys.
Other objects and advantages of this invention will be better appreciated from the following detailed description.